1. Field of the Invention
This invention relates to an image forming apparatus having a plurality of optical-information exposure positions on an electrophotographic photosensitive member, and an image forming apparatus having a device for directly supplying electric charges on a dielectric member, in which deviation in superposed positions of a plurality of images formed on a latent-image bearing member, comprising a photosensitive material, a dielectric material or the like, is prevented.
2. Description of the Related Art
In conventional image forming apparatuses, in which a plurality of optical-information exposure positions are provided using a laser optical system or a digital optical system, comprising LED (light-emitting diode) devices and the like, and color toner images corresponding to the exposure of respective optical information obtained after development are superposed, in order to precisely superpose respective color toners, the timing to start exposure by exposure means at respective exposure positions must, of course, be adjusted. In particular, it is preferred to maintain the moving speed of the photosensitive member during image formation at a constant value.
Accordingly, in a driving device for the photosensitive member, a high-precision driving motor and complicated electric rotation control are required. Furthermore, high rotational accuracy, high roundness and high mounting accuracy are also required for driving transmission elements, such as gears, pulleys and the like.
In particular, digital optical-information exposure means utilizing laser light or LED-device arrays have recently been adopted in many image forming apparatuses, because an output from a computer can be used and electrical image processing can be easily performed. Such an exposure means performs exposure using a small light spot with a very small exposure width (for example, 63.5 .mu.m pitch with 400 dpi (dots per inch), 42.3 .mu.m pitch with 600 dpi, or the like) in the main scanning direction (the direction orthogonal to the moving direction of the photosensitive member) on the photosensitive member. Accordingly, when a plurality of toner images are superposed, deviation between superposed images causing unevenness in the density of a synthesized image in the moving direction of the photosensitive member becomes very pronounced.
Furthermore, when the moving (circumferential) speed of the photosensitive member at the exposure position for forming a latent image on the photosensitive member differs from the speed of the photosensitive member when the toner image obtained by developing the latent image with a toner is transferred onto a transfer material, the following problems arise.
That is, when a linear electrostatic latent image with an equal interval is formed on the photosensitive member in the main scanning direction orthogonal to the moving direction of the photosensitive member, and the formed latent image is reproduced as a toner image on a transfer material, suppose that a speed difference is produced between the exposure position of the photosensitive member and the transfer position. In the case of "the speed at the exposure position&gt;the speed at the transfer position", a lengthened reproduced image compared with the actual image is formed, as shown in FIG. 9. In the case of "the speed at the exposure position&lt;the speed at the transfer position", a shortened reproduced image compared with the actual image is formed, as shown in FIG. 10.
If periodicity is present in the moving speed of the photosensitive member, an image as shown in FIG. 11, in which periodic unevenness in pitch and density is present in the moving direction of the photosensitive member, is reproduced.